I've read the forums and TechNet articles and was able to successfully enable Bitlocker (Used Space Only Encrypted) on Windows 8.1 PC's. I used MDT 2013 (Setup up Bitlocker settings in the customsettings.ini) to pre-provision Bitlocker in WinPE 5.0 and then at the end of the MDT task sequence it enables bitlocker and sends the recovery key to AD. Works great and avoids the hassle of waiting for the entire drive to encrypt.
We have Windows 7 PC's in our environment and using the same task sequence settings, appears to enable the Used Space Only encryption in WinPE however once the OS is installed and you check the Bitlocker Status (Manage-bde -status) it shows that the drive is Fully Encrypted. Once the Task Sequence enables bitlocker, a recovery key is sent to AD successfully and protection is on. It appears to have the same result as with Windows 8.1 Computers however the conversion status reads "Fully Encrypted" versus "Used Disk Space Only". The Windows 7 PC's have a 500 GB Hard Drive and I know they would not fully encrypt that quickly. Are these Windows 7 PC's using "Used Space Only" encryption but reporting "Fully Encrypted" because maybe Windows 7 doesn't know how to report used space only since it is a Windows 8 bitlocker feature?